1. Field of the Invention:
This invention relates to isocyanate functional copolymers which permit low temperature crosslinking of curable compositions.
2. Description of the Prior Art:
U.S. Pat. No. 3,290,350 discloses the copolymerization of isopropenyldimethylbenzylisocyanate with ethylene using free-radical type initiators. The resultant copolymers are of poor solubility in benzene and low isocyanate content.
German Patent 1,745,279 describes in Example 5 the azo catalyzed copolymerization of p-TMI with methyl acrylate at a low 82% conversion.
U.S. patent application Ser. No. 499,961 filed Jun. 1, 1982, and now abandoned, describes copolymers of meta- or para-isopropenyl-.alpha.,.alpha.-dimethylbenzyl-isocyanate (hereinafter, "m/p TMI") with certain unsaturated comonomers. The text of patent application Ser. No. 499,961 has been published by the European Patent office as EP 0 130323 A2. The primary technical disclosure of Ser. No. 499,961 is the use of a relatively high molecular weight isocyanate polymer (e.g., M. W. 6400-17400) in conjunction with a low molecular weight polyhydroxy compound (e.g., diethylene glycol, 1,2,6-hexanetriol, trimethylolpropane). Ser. No. 499,961 exemplified copolymers were not able to successfully cure in combination with low molecular weight polyols at room temperature within a period of four days. The single instance of room temperature cure recited in Ser. No. 499,961 was for a high molecular weight terpolymer "Y" containing 15 mole percent m-TMI, 40 mole percent butylacrylate, and 45 mole percent methylmethacrylate cured with trimethylolpropane cured in seven days using unconventionally high levels of tin curing catalyst (viz., 1% organotin compound based on weight of terpolymer solids). Moreover, at room temperature, the formulation of terpolymer "Y" and trimethylolpropane was not homogeneous.
Patent application Ser. No. 563,882 filed Dec. 21, 1983, now abandoned, and copending patent application 07/113,337 filed Oct. 22, 1987 are continuations-in-part of Ser. No. 499,961 and describe copolymers of m/p TMI with various unsaturated comonomers. These patent applications contain a single exemplification of a seven day room temperature cure.
Subsequent experimentation has failed to achieve a consistent 4 day or less room temperature cure by following the examples or general teaching of Ser. No. 499,961 or its continuation-in-part applications.
None of the prior art copolymers of m/p-TMI possess all of the combined advantageous properties required for a practical isocyanate functional copolymeric crosslinking agent.
It is desirable to prepare m/p TMI copolymers which have the following collective advantages over the prior art:
1. Room temperature curable in 4 days or less.
2. High isocyanate functionality (over 40 mole % m/p TMI)
3. High Organic solvent solubility
4. Low molecular weight
5. Ability to cure polyhydroxy, carboxy, or amino bearing materials.
6. Low residual m/p TMI.